Quadrupole resonance (QR) allows noninvasive, short-range detection of analytes containing nitrogen, including many explosives. Unlike other technologies, a QR detection system can discriminate among different types of explosives and distinguish them from benign nitrogen compounds, because the QR response from each nitrogen compound has a distinct spectral signature.
There is a great current need to detect explosives concealed within containers, such as luggage, mail, improvised explosive devices, and minimal metal landmines. At the present time, x-ray detection is the primary technology used at aviation security checkpoints. X-ray detection reveals the presence and shape of objects that absorb energy from the x-ray beam, but cannot distinguish between benign material and explosive devices.
Current QR detection systems are hindered by problems such as inadequate sensitivity, limited operating temperature range, and electrical interference from both internal and external RF sources. The signal-to-ratio (SNR) of a nuclear resonance measurement using QR is proportional to the square-root of the Quality-factor (Q-factor) of the probe. Q-factors are approximately 100 for normal (non superconducting) metal coils, which severely limits the SNR.
The frequency of the QR response from a particular nitrogen compound is temperature dependent, and existing commercial QR detection systems requires that the searched objects be held within small temperature range. Further, the small QR response is easily masked by RF sources, such as AM broadcast stations and engine ignition noise, that are external to the detection system.
Excitation of a QR response requires the application of a pulsed RF magnetic field within the search volume. The applied RF pulse may excite spurious responses from materials within the search volume that can obscure the QR response, leading to an unacceptably large false alarm rate. Examples of internal noise sources include the decaying magnetic field generated by currents induced within conductive materials located within the search volume, as well as piezoelectric responses from materials within the search volume.